Almost half of the world's farmland has at least 10 percent tree cover, according to a study on Monday indicating that farmers are far less destructive to carbon-storing forests than previously believed.

"The area revealed in this study is twice the size of the Amazon, and shows that farmers are protecting and planting trees spontaneously," Dennis Garrity, Director General of the World Agroforestry Center in Nairobi, said in a statement.

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The Centre's report, based on satellite images and the first to estimate tree cover on the world's farms, showed tree canopies exceeded 10 percent on farmland of 10 million square kms (3.9 million sq miles) -- 46 percent of all agricultural land and an area the size of Canada or China.

By one yardstick used by the U.N.'s Food and Agricultural Organization, a "forest" is an area in which tree canopies cover at least 10 percent of an area. The definition excludes, however, farmland or urban areas.

The report said that farmers keep or plant trees for uses such as production of fruit, nuts, medicines, fuel, building materials, gums or resins. Trees also provide shade for crops, work as windbreaks, boundary markers or to help avert erosion.

Farms are often portrayed as enemies of forests -- homes to a wide diversity of animals and plants. Forests are also giant stores of heat-trapping carbon dioxide, the main greenhouse gas.

The report found that trees were integral to agricultural landscapes in all parts of the world, with the exceptions of arid North Africa and West Asia.